From 704cf451cbac9738e187bfb91abe244a56c46059 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Mon, 16 Dec 2019 14:34:26 +0100
Subject: [PATCH] 2019: d16: ex2: add solution

---
 2019/d16/ex2/ex2.py | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)
 create mode 100755 2019/d16/ex2/ex2.py

diff --git a/2019/d16/ex2/ex2.py b/2019/d16/ex2/ex2.py
new file mode 100755
index 0000000..a13a0bf
--- /dev/null
+++ b/2019/d16/ex2/ex2.py
@@ -0,0 +1,29 @@
+#!/usr/bin/env python
+
+import sys
+from functools import reduce
+from itertools import chain, cycle
+from typing import Iterable, List
+
+
+def main() -> None:
+    rep = 10000
+    signal = [int(d) for d in sys.stdin.read().strip()] * rep
+    offset = reduce(lambda a, b: a * 10 + b, signal[0:7])
+
+    assert offset >= len(signal) / 2  # Sanity check
+    # The trick is that the second half is only affected by itself (triangular matrix):
+    # For i > len(signal) / 2, new_signal[i] = sum(signal, i, len(signal))
+    # Therefore, we're only interested in numbers that start at the offset
+    signal = signal[offset:]  # Only take the end we need
+
+    for __ in range(100):
+        for i in range(len(signal) - 1, 0, -1):  # Do the sum from the end
+            signal[i - 1] += signal[i]
+            signal[i - 1] = signal[i - 1] % 10
+
+    print(reduce(lambda a, b: a * 10 + b, signal[:8]))
+
+
+if __name__ == "__main__":
+    main()